In present-day drilling operations it is advantageous to have the capability of controlling the directional course of the drill bit as it progressively excavates a borehole. Such controlled directional drilling is particularly needed in any offshore operation where a number of wells are successively drilled from a central platform to individually reach various target areas that are respectively situated at different depths, azimuthal orientations and horizontal displacements from the drilling platform. It should, of course, be recognized that directional drilling is not limited to offshore operations alone since there are also many inland operations where the drill bit must be deliberately diverted in a desired lateral direction as the borehole is being drilled.
Heretofore most directional drilling operations were carried out by temporarily diverting the drill bit in a selected direction with the expectation being that the drill bit would thereafter continue to advance along a new course of excavation when normal drilling was resumed. For instance, in a typical whipstock operation, a special guide is temporarily positioned in a borehole to guide a reduced-size drill bit as it drills a short deviated pilot hole in a selected direction. The guide device is then removed and drilling is resumed with a full-size drill bit for reaming out the pilot hole and continuing along the new course of excavation established by the pilot hole. Similarly, in another common directional drilling technique, a so-called "big eye " drill bit is selectively oriented in a borehole to direct an enlarged port in the bit in a given lateral direction. Then, while rotation of the bit is temporarily discontinued, the mud pumps are operated for forcibly discharging a jet of drilling mud from the enlarged port to progressively carve out a cavity in the adjacent sidewall of the borehole into which the bit will hopefully advance whenever rotation is resumed. A third common directional drilling technique employs a fluid-driven motor and earth-boring device that are coupled to a so-called "bent sub" which can be cooperatively controlled from the surface for selectively positioning the device to drill along any one of several courses of excavation.
With these typical directional drilling techniques, it is necessary to make directional measurements from time to time so that appropriate and timely corrective actions can be taken whenever it appears that the drilling apparatus is not proceeding along a desired course of excavation. Nevertheless, when typical wireline measuring techniques are employed, the course of the drilling apparatus can not be determined without periodically interrupting the drilling operation each time a measuring tool is lowered into the drill string to obtain directional measurements. Thus, when wireline measuring techniques are being used, it must be decided whether to continue drilling a given borehole interval with a minimum of delays or to prolong the drilling operation by making frequent directional measurements to be certain that the drilling apparatus is maintaining a desired course of excavation.
With the advent of various measuring-while-drilling or so-called "MWD" tools such as those which are now commercially available, it became possible to transmit to the surface one or more directional measurements either separately or in conjunction with other real-time downhole measurements without having to interrupt the drilling operation. Generally these directional measurements are obtained by arranging a MWD tool to include typical directional instruments adapted to provide real-time measurements representative of the spatial position of the tool in a borehole. Alternatively, as described in U.S. Pat. No. 2,930,137 to Jan J. Arps, it has been proposed to arrange a typical MWD tool with special instrumentation for measuring the bending moments in a lower portion of the drill string to provide real-time measurements which are presumably representative of the crookedness or curvature of the borehole as it is being drilled.
Accordingly, when a conventional drill bit is combined with a MWD tool which can provide either or both of these realtime measurements, it can be determined whether at least limited downhole directional changes are being effected from the surface by varying one or more drilling parameters such as the rotational speed of the drill string, the flow rate of the drilling mud in the drill string and the load on the drill bit. The ability to make these real-time directional or bending-moment measurements has also made it feasible to combine either a big-eye bit or a drilling motor coupled to a controllable bent sub with a suitable MWD tool for continuously monitoring the directional drilling tool as it excavates a borehole. It should be noted in passing that it has been found advantageous to employ MWD tools capable of providing real-time directional measurements while drilling a deviated borehole or while drilling a borehole along a generally-vertical course of excavation.
Regardless of the type of drilling apparatus that is employed, the instrumentation section of a typical MWD tool is ordinarily separated from the drilling apparatus by various tool bodies and, in some instances, one or more drill collars as well. Accordingly, when a directional measurement is made, the drilling apparatus is already at an advanced location that the measuring instruments will not reach until perhaps several hours later. -n other words, any particular directional measurement represents only the previous location of the drilling apparatus when it was drilling the borehole interval that is presently occupied by the directional instrumention in the MWD tool. Since the several interconnecting bodies and drill collars are relatively flexible, the drilling apparatus can be easily diverted from its intended course of excavation by such things as variations in formation properties or in the borehole environment or by changes in the performance characteristics of the drilling apparatus. Even when such factors are taken into account, it can not be realistically assumed that the drilling apparatus will always remain axially aligned with the instruments in the MWD tool. Thus, it must be recognized that these prior-art bending-moment and directional measurements can at best provide only an estimate of the probable location of the drilling apparatus at the time that a particular measurement was made. With so many variables, those skilled in the art will, of course, appreciate that these prior-art bendingmoment and directional measurements can not be reliably used for accurately determining the present position of the drilling apparatus much less predicting the future course of excavation of the drilling apparatus.
Accordingly, it was not until the invention of the new and improved methods and apparatus that are described in U.S. Pat. Nos. 4,303,994 and 4,479,564 to Denis R. Tanguy that it was considered possible to determine the position of the drilling apparatus with some degree of accuracy as well as to predict its future course of excavation. It will, of course, be recognized that the teachings of these two Tanguy patents can be useful for maintaining an earth-boring device on a particular course of excavation as well as for selectively redirecting the boring apparatus as necessary to reach a designated target area. Nevertheless, despite the advantages of employing the principles of the aforementioned Tanguy patents, there are situations in which the future course of excavation of earth-boring apparatus must be ascertained with more precision than would be possible by practicing the inventions disclosed in those patents.